Numerical investigation on stall flutter of an airfoil with split drag rudder

This paper investigates the stall flutter characteristic of a National Advisory Committee for Aeronautics (NACA) 0012 airfoil with a split drag rudder (SDR). The effect of split angle and initial disturbance on stall flutter properties are studied by the computational fluid dynamics (CFD)/computational structure dynamics (CSD) coupling method. The inherent flow mechanism is illustrated with the vorticity and pressure distribution contours. The results show that stall flutter occurs when the pitch motion amplitude of the response exceeds the pitch instability branches of the energy maps. The increase in the split angle leads to an increase in the additional stiffness of the aerodynamic forces so that the amplitude of the pitching motion induced by the initial disturbance decreases and the bifurcation speed increases with the split angle. The trailing edge vortex and the standing vortex between the rudders form affected by the SDR, which reduces the nose-up moment of the airfoil at small angles of attack and increases the nose-down moment after stall occurs. It enables airfoil to obtain more energy from the flow field so that the LCO oscillation amplitude increases. Moreover, it is found that the SDR-induced trailing-edge vortices will excite a large-amplitude stall flutter of an airfoil with a zero initial angle of attack and disturbance.